A power supply device, such as a battery, needs to be controlled when the battery is charged and when the battery is discharged (i.e., electrical current is supplied to the load) to prevent overcharging and overdischarging of the battery. Therefore, bidirectional semiconductor switches that can turn ON and OFF an AC signal or AC power are necessary. A composite bidirectional device consisting of a reverse parallel combination of unidirectional semiconductor devices has been used as such a bidirectional semiconductor switch. Furthermore, it has been attempted to provide a miniaturized power supply device using a power IC consisting of a semiconductor substrate on which such a composite directional device and an IC for controlling it are integrated. In addition, a single (i.e., noncomposite) bidirectional device has been developed. In this respect, a bidirectional lateral insulated gate bipolar transistor (LIGBT) has been proposed in ISPSD (International Symposium on Power Semiconductor Devices and ICs) 1997, pp. 37-40, for example. The structure and operation of the bidirectional LIGBT are described below.
FIG. 30 is a cross-sectional view of main portions of the bidirectional LIGBT. In the bidirectional LIGBT, two p+ well regions 504 and 505 are formed on the surface side of an n-type semiconductor layer 503, and n+ emitter regions 506 and 507 are formed in the p+ well regions 504 and 505, respectively. The p+ well regions 504 and 505 are formed with their surfaces exposed on the surface of the n-type semiconductor layer 503, and are spaced from each other by a given or predetermined distance (drift distance) to maintain a given breakdown voltage. Furthermore, the n+ emitter regions 506 and 507 are formed with their surfaces exposed on the surface of the n-type semiconductor layer 503 (i.e., the surfaces of the p+ well regions 504 and 505).
Insulated-gate type gate electrodes 510 and 511 consisting of polysilicon are formed over the portion located between the two n+ emitter regions 506 and 507 of the p+ well regions 504 and 505 via gate insulator films 508 and 509. Furthermore, emitter electrodes 512 and 513 are formed to bridge across both the p+ well region 504 and the n+ emitter region 506 and across both the p+ well region 505 and the n+ emitter region 507. In this configuration, the main current flowing in both directions between the emitter electrodes 512 and 513 can be controllably turned ON and OFF by controlling the voltage applied to the gate electrodes 510 and 511.
FIG. 31 is a diagram showing the output characteristics of the bidirectional LIGBT of FIG. 30. Since the main current does not begin to flow unless the voltage exceeds the rising voltage (0.6 V) arising from the internal potential of the pn-junction, the ON voltage is high and ON loss is large at a small current region. As an improvement of the configuration of FIG. 30, a single bidirectional MOSFET comprising a bidirectional device consisting of a MOSFET whose voltage decreases down to 0 V when the device is started to be operated is described, for example, in JP-A-11-224950, which is described below.
FIG. 32 is a cross section of main portions of the related art bidirectional MOSFET. Here, a bidirectional LDMOSFET (lateral double-diffused MOSFET) is shown as an example. In the same way as the above-described example, this transistor has an SOI structure. An n-type semiconductor layer 103 is formed over a semiconductor substrate 101 with an insulator layer 102 formed therebetween. Two n++ drain regions 104 and 105 are formed on the surface side of the n-type semiconductor layer 103. A p+ well region 106 is formed between the n++ drain regions 104 and 105. The p+ well region 106 is formed to a depth reaching the insulator layer 102, dividing the n-type semiconductor layer 103 into two regions. Two n++source regions 107 and 108 are formed in the p+ well region 106. A p++ base contact region 109 is formed between both the n++ source regions 107 and 108. The n++ drain regions 104, 105 and the p+ well region 106 are exposed on the surface of the n-type semiconductor layer 103. The n++ source regions 107, 108 and p++ base contact region 109 are exposed on the surface of the p+ well region 106. Insulated-gate type gate electrodes 112 and 113 are formed over the p+ well region 106 with gate insulator films 110 and 111 formed therebetween. The gate electrodes 112 and 113 are connected together. Drain electrodes 114 and 115 are connected with the n++ drain regions 104 and 105, respectively. A source electrode 117 is connected across both the n++source region 107 and the p++ base contact region 109 and across both the n++ source region 108 and the p++ base contact region 109.
To turn ON the aforementioned bidirectional LDMOSFET, a voltage is applied between the gate electrode 112 and the source electrode 117 and between the gate electrode 113 and the source electrode 117 such that the gate electrodes 112 and 113 are placed at a positive potential. At this time, channels are formed immediately under the gate insulator films 110 and 111 in the p+ well region 106. If it is assumed that a voltage is applied between the drain electrodes 114 and 115 to place the drain electrode 114 at a higher potential, an electron current flows from the drain electrode 114 to the drain electrode 115 via the n++ drain region 104, the n-type semiconductor layer 103, the channel corresponding to the gate electrode 112, the n++source region 107, the source electrode 117, the n++ source region 108, the channel corresponding to the gate electrode 113, the n-type semiconductor layer 103, and the n++ drain region 105 in this order. At this time, the electron current dominates the electrical current, i.e., unipolar. Since there is no junction in the current path, no offset component is produced even at low potentials. That is, the linearity becomes good even at very small currents. Where the polarity of the voltage applied between the drain electrodes 114 and 115 is reversed, the sense of the current is reversed but the operation is similar. As a result, as shown in FIG. 33, an AC current can be supplied. It also can be expected that operation of good linearity occurs even at very small currents.
On the other hand, in order to turn OFF the above-described bidirectional LDMOSFET, the gate electrodes 112 and 113 are shorted to the source electrode 117. This annihilates the channels formed immediately under the gate insulator films 110 and 111 in the p+ well region 106. The electron current no longer flows and hence the device is turned OFF. In the OFF state, no current flows even if either positive or negative voltage is applied between the drain electrodes 114 and 115. That is, the device takes an OFF state for AC voltage. Under this state, the breakdown voltage is equal to the breakdown voltage of a half portion of the bidirectional LDMOSFET.
The AC power can be turned ON and OFF with one chip using the bidirectional LDMOSFET. Furthermore, during conduction, the linearity of the voltage-to-current characteristics is good even at very small currents. The device can be used to turn ON and OFF a signal current. In addition, the gate electrodes 112 and 113 are connected together, and there is only one source electrode 117. Therefore, only one driver circuit is necessary to supply a control signal to the gates. Consequently, the device becomes easy to control.
As mentioned previously, the main current flows through the channels without via a pn junction. Therefore, the main current is fundamentally the same as the current flowing through a resistor. The current flows at zero or higher voltage. The ON voltage is small at small currents, and thus the ON loss can be reduced. However, in the bidirectional LDMOSFET of FIG. 32, the breakdown voltage is sustained by one MOSFET of the bidirectional LDMOSFET. Therefore, to sustain forward-reverse breakdown voltages, both MOSFETs are required to show breakdown voltage. Hence, the area occupied is doubled. This increases the area occupied between the drain regions. Furthermore, the planar structure makes it difficult to reduce the size of cells forming the bidirectional LDMOSFETs. Accordingly, it is difficult to improve the ON voltage.
Accordingly, there still remains a need for a semiconductor device with high breakdown voltage, with the reduced ON voltage without the foregoing problems and while increasing the cell density of the bidirectional device.